Compositions based on poly(1,4-cyclohexylenedimethylene terephthalate), PCT, are useful as injection molding compounds for applications such as electrical/electronic parts, automotive parts, and mechanical parts, owing to their excellent physical properties, chemical resistance, and heat resistance. The heat resistance of PCT compositions is higher than that which can be obtained with common thermoplastic polyesters such as poly(ethylene terephthalate), PET, and poly(butylene terephthalate), PBT. However, for thin-walled parts, PCT is deficient in crystallization rate and melt flow. Further, the high processing temperature needed for PCT causes the molecular weight of the polymer to degrade during injection molding, which in turn causes a decrease in physical properties. These problems prevent the wider use of PCT as a molding material.
Solutions suggested to the first problem, that of low crystallization rate, have included the use of long chain alcohols (U.S. Pat. No. 4,859,732), amide compounds (U.S. Pat. No. 4,894,404) and toluene sulfonamide (Japan Kokai H2-187451). Normal plasticizers do not provide sufficient increase in the crystallization rate for good processability. Also, they are volatile and thus cause problems in mold corrosion, charring due to volatile gases, and loss of physical properties.
Regarding the problem of degradation during injection molding, it has been previously disclosed to add epoxy compounds to a similar terephthalate-based polyester, PET. It has also been previously disclosed to add trifunctional phenol-type epoxy compounds, trifunctional isocyanuric acid-based compounds (EP-A-0273149), phenoxy resin (U.S. Pat. No. 4,837,254) and glycidyletherester compounds (Japan Kokai H3-9948) to PET.
The use of certain epoxy compounds in PCT is disclosed in EP-A-273149. This application also discloses the use of a phosphate compound as a component of a formulation. Phosphates are not within the scope of the present invention. The use of other epoxy/phosphorous combinations are disclosed in WO 9106602.
The addition of stabilizers to a blend of a PCT-type copolyester and polycarbonate has been disclosed (Japan Kokai S62-270653).
The use of heat stabilizers to avoid color formation during heat aging of PCT copolyesters (EP-A-0328528) is also known.
Known epoxy compounds have sometimes been effective in avoiding a drop in apparent viscosity due to degradation. However, it is difficult to avoid a loss of physical properties when a drop in molecular weight occurs. It is also difficult to obtain stable processing performance, for example, during injection molding. This is due to the polymer branching which occurs as a result of the continuing reaction between the multifunctional epoxy and the polymer melt in the injection molding machine. This branching causes changes in the flow characteristics of the polymer. Thus it is not possible to obtain sufficient melt stability using only the known epoxies.
Unexpectedly, a suitable plasticizer for PCT has been discovered which improves crystallization rate and melt flow without lowering physical properties and which has a much lower level of volatile off-gas.
Further, use of specific phosphorous compounds with this plasticizer gives synergistic improvements in melt stability. Use of combinations of the plasticizer with phosphorous compounds and with multifunctional epoxy compounds dramatically improves the physical properties, especially after exposure of the PCT in a molten state to long residence times in an injection molding machine.